This invention relates to an air control device for regulating the amount of air flowing through an air passage bypassing a throttle valve during engine warming-up.
In an electronically controlled fuel-injection type internal combustion engine, an air passage bypassing a throttle valve is provided to connect the upstream side of the throttle valve with the downstream side thereof in such a way that the air flow through the air passage is controlled during engine warming-up. That is, the effective area of the air passage for the air flow is controlled to increase the air flow rate so that the engine speed can increase thereby to advance engine warming-up.
FIG. 1 shows a conventional mechanism for controlling the effective area for the air flow through the air bypassing passage. An air passage A has a passage inlet A' leading to the upstream side of a throttle valve (not shown) and a passage outlet A" leading to the downstream side thereof. A plate-like shutter B is rotatably set in a plane at a right angle to the air passage A. The shutter B has a slit C formed vertically therein so that the rotation of the shutter B controls the air flow in cooperation with the wall of the air passage A. A bimetallic element E is actuated in response to engine temperature so as to rotate the shutter B in a given direction.
In such a conventional structure, the shutter B is pulled by the vacuum in the outlet A" so that the friction between the shutter B and a sliding portion F of the wall of the air passage A increases. In particular, when the vacuum in the passage outlet A" is strong, the shutter B can not rotate smoothly due to the friction between the shutter B and the sliding portion F. If the closing force of the bimetallic element E is increased to compensate for this frictional resistance, then when engine speed is decreased with an accompanying decrease of vacuum and frictional resistance, the shutter B will have a tendency to snap shut, making it difficult to achieve precise control of air flow.